Belt drives in a power train of a motor vehicle, in particular, in a pulley plane of a combustion engine opposite the transmission side, have long been known. In this case, the crankshaft drives ancillary units by way of the belt drive, for example, an electric generator, an air conditioner compressor, oil and/or water pumps and the like. To be able to design a so-called starter-generator, to be utilized as an electric generator and a starter motor, in terms of its size and performance for both functions, belt drives with planetary gear sets are proposed in German Patent Application No. 10 2004 060 991 and German Patent Application No. 10 2007 021 233 which set different transmission ratios depending on the flow of torque between the rotor of the starter-generator and the crankshaft of the combustion engine. For example, during the process of starting the combustion engine, the speed of the starter-generator is translated to a lower speed in order to attain a necessary angular momentum for an impulse start, while in generator mode little or no transmission ratio translation is provided between rotor and crankshaft.
The shifting of the transmission ratios and of the flow of torque is accomplished by means of one or more actuators, where on the one hand the gear ratio is achieved by means of a brake that connects the ring gear firmly to the housing and on the other hand the flow of force is achieved by means of two opposing switchable free-wheeling mechanisms which are switched contrary to each other, so that depending on the blocking switching of one of the two free-wheeling mechanisms the rotor can overrun the crankshaft or the crankshaft can overrun the rotor. In this way, with transmission ratio reduction the idle combustion engine can be started, and with the same transmission ratio it can be operated with torque support from the starter-generator (boost mode). Furthermore, the starter-generator can be driven as a generator by the combustion engine at both transmission ratios. Moreover, it can provide stationary air conditioning of the motor vehicle with the combustion engine not running, by driving an ancillary unit provided as an air conditioner compressor by the starter-generator, preferably without a transmission ratio change, with the crankshaft over-run.
In this case, it is difficult to control the brake that shifts the transmission ratios, and must take place precisely in order to avoid jerky changes in torque that can result in material wear and loss of comfort.